Tone control utilizing an operational amplifier with differential inputs

ABSTRACT

A tone control circuit of a design simpler and more effective than that of those hitherto known, built up of an operational amplifier with differential input, which at the low and/or high tones and/or over a single section, or several, of the audio frequency band permits a transmission characteristic departing from the straight line which in a manner differing from similar known methods guarantees the monotonous decay of the transmission characteristic on the edges of the audio frequency band. Unlike conventional solutions, monotonous decay is achieved by inserting in series with a reactance connected between the slide and the earth point of the potentiometer connected between the inverting and non-inverting points of the operational amplifier an ohmic resistance, whereas according to need a differentiating or integrating RC circuit is connected between the controlling signal source and the non-inverting point, as the case may be.

United States Patent Huszty et al.

[ 1 June 11, 1974 1 TONE CONTROL UTILIZING AN OPERATIONAL AMPLIFIER WITH DIFFERENTIAL INPUTS [75] Inventors: Denes Huszty; Karoly Szabados,

both of Budapest, Hungary [73] Assignee: Elektroakusztikai Gyar, Budapest,

Hungary [22] Filed: Sept. 6, 1972 [21] Appl. No.: 286,680

[30] Foreign Application Priority Data Feb. 15, 1972 Hungary EE 2004 [52] US. Cl. 179/] D, 330/69 [51] Int. Cl H04r 3/00 [58] Field of Search 179/] D, 1 E; 333/28 T;

[56] References Cited UNITED STATES PATENTS 3,519,947 7/1970 Thelen 330/185 3,629,721 12/1971 Fordyce.....

3,646,464 2/1972 Boggs 330/109 OTHER PUBLICATIONS Philbrick/Nexus Research, Teledyne Company, Operational Amplifiers, 8/1969, Section 111.26 and Section 111.70.

Primary Examiner-l(athleen H. Claffy Assistant Examiner.lon Bradford Leaheey Attorney, Agent, or Firm-Young & Thompson 5 7] ABSTRACT A tone control circuit of a design simpler and more effective than that of those hitherto known, built up of an operational amplifier with differential input, which at the low and/or high tones and/or over a single section, or several, of the audio frequency band permits a transmission characteristic departing from the straight line which in a manner differing from similar known methods guarantees the monotonous decay of the transmission characteristic on the edges of the audio frequency band.

Unlike conventional solutions, monotonous decay is achieved by inserting in series with a reactance connected between the slide and the earth point of the potentiometer connected between the inverting and non-inverting points of the operational amplifier an ohmic resistance, whereas according to need adifferentiatingor integrating RC circuit is connected between the controlling signal source and the non-inverting point, as the case may be.

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smasel PAVENFEMM w #914 SHEET'110F12 amassl sum 12 or 12 .illii u 1 m4 TONE CONTROL UTILIZING'AN OPERATIONAL AMPLIFIER WITH DIFFERENTIAL INPUTS The subject-matter of the invention is a tone control circuit of a design simpler and more effective than that of those hitherto known, built up of an operational amplifier with differential input, which circuit at the low and/or high tones, and/or over any section or sections of the audio frequency band permits a transmission characteristic departing from the straight line in a manner such that on the edges of the audio frequency band the transmission characteristic is decaying monotonously.

The tone control circuit has important functions to perform in the electroacoustic transmission chain. As a matter of fact by means of the tone control circuit the transmission characteristic of the complete transmission chain may be compensated in a manner such that in a definite frequency band dependent on the frequency this characteristic takes on a definite, e.g. straight-line character.

In the following text reference to literature will be distinguished by numbers in parentheses. For sake of easier study the list of references follows after the specification and claims.

Experience teaches that owing to the complicated interaction of sound radiator and room the transmission characteristic of the electroacoustic chain is in all cases uneven (l). The unevennesses are partly in the nature of resonance, i.e., within the region of a narrow frequency band the characteristic presents an acute peak or decay, partly presenting a rise or decay, as the case may be, extending over a relatively wide frequency band. The unevennesses of the transmission characteristic extending over a wide band are normally compensated by tone control circuits including passive (2) or active elements (3). For the compensation of the sharp peaks or fluctuations of the characteristic a network built up of passive elements, consisting of parallel oscillating circuits in series, is recommended (4). Each of the known methods presuppoeses the use of a large number of components, and also the adjustment of the circuits is of extreme complexity.

In the design of the tone control circuit the appearance of circuits built up with operational amplifiers of differential input indicated a remarkable improvement.

Of these circuits then the bass and high tone control circuits were built up. In the circuits of this type which stood the test best an operational amplifier of differential input controlled at its non-inverting point was used. An ohmic resistance was inserted between the inverting point and the earth point, and also between the noninverting point and the earth point. In like manner an ohmic resistance was connected between the inverting point and the output point of the operational amplifier. Between the inverting and non-inverting points a potentiometer was connected, whose slide was for low tone control connected to the earth point through the inductance and condenser in series, whereas for high tone control only the condenser has been inserted. An ohmic resistance was connected between the controlling signal source and the non-inverting point.

A critical deficiency of a low tone control circuit of this design is that below the lower limit of the useful audio frequency band, i.e., below about 40 Hz, the amplifier is devoid of band limitation, and therefore transmits signals of very low frequencies, even direct current, with the same gain, i.e., a gain of unity. ln this way the amplifier transmits the noise output of the signal sources connected before the circuit of below the lower limit of the audio frequency band with a gain of unity and consequently disturbs the gain of the useful signals. Another deficiency of the known methods is that at acceptable costs the tolerances of the capacity and return loss of the high-capacity condenser in the system are in general very high, and consequently the scattering of the values of both the resonance frequency and the unevenness producible on the resonance frequency is also extreme.

A drawback of the high-tone control circuit here referred to is that at the upper limit of the useful audio frequency band, i.e., for signals of a frequency in excess of l6 kHz the gain or loss of the circuit varies with a slope of respectively +6 dB/octave and -6 dB/octave, whereas with the slide of the potentiometer in midposition, in a wide band this value will be constant for frequencies higher than the upper cut-off frequency of the audio frequency band. Consequently since there is a band limitation only when the slide of the potentiometer is in the neighborhood of the non-inverting input point, the method hitherto known will in other positions of the potentiometer slide appreciably amplify the disturbing noise voltage arising in the section of the electro-acoustic chain preceding the circuit. Moreover, the components of an out-of-band frequency of the noise voltage will owing to the cross-modulation caused by the insignificant, yet inevitable non-linear property of the amplifier step up the noise voltage within the useful band, too. i

The present invention eliminates the deficiencies and drawbacks of both the low-tone and high-tone control circuits.

With the tone control circuit according to the invention, which contains at least one operational amplifier of a differential input, controlled at its non-inverting point, with an ohmic resistance inserted between the inverting and earth points and also between the noninverting and earth points, further with another ohmic resistance connected between the inverting point and the output point of the operational amplifier, while there is a potentiometer connected between the inverting and non-inverting points, with a slide via a reactance connected to the earth point, while there'is an ohmic resistance connected between the controlling signal source of theoperational amplifier and its noninverting point, this is achieved in a way such that an ohmic resistance is connected in series together, with an inductance or condenser, with the reactance inserted between the slide of the potentiometer and the earth point, whereas differentiating and integrating RC circuits are inserted between the controlling signal source and the non-inverting point.

Other details and advantages of the tone control circuit according to the invention are, as compared to other known circuits of a similar type, made clear by a consideration of the following embodiments and the accompanying drawings. In the particular figures corresponding components have been distinguished by uniform symbols of reference. in the graphs showing the transmission characteristics the values of the frequencies are indicated on the abscissa, those of the decibels on the ordinate.

FIG. 1 presents the circuit diagram of a known lowtone control circuit.

FIG. 2 displays the transmission characteristic associated with the previous tone control circuit.

FIG. 3 shows the circuitry of a low-tone control circuit according to the invention.

In the diagram in FIG. 4 the transmission characteristic of the set-up according to FIG. 3 has been plotted.

FIG. 5 shows the circuitry of a known high-tone control circuit.

FIG. 6 displays the characteristic of the set-up according to FIG. 5.

In the graph in FIG. 7 the transmission characteristic demonstrating the effects of the new technique used in the set-up according to FIG. 5 has been plotted.

FIG. 8 presents an embodiment of the high-tone control set-up according to the invention.

FIG. 9 displays the transmission characteristic of the set-up according to FIG. 8.

FIGS. 10 and 11 present each a variant of the set-up according to FIG. 8.

FIG. 12 present a possible embodiment of the circuitry of the combined lowand high-tone control circuit according to the invention.

FIG. 13 displays the transmission characteristic of the set-up according to FIG. 12.

In FIG. 14 a set-up has been presented where in addition to general lowand high-tone control local peak or hole control may be performed.

In the graph in FIG. 15 the transmission characteristic associated with the set-up of FIG. 14 has been plotted.

In the graph in FIG. 17 details of the transmission characteristic associated with FIG. 16 have been displayed.

In FIGS. 18 and 19 an embodiment of an improved variant of the tone control circuit according to the invention has been displayed.

In the following an explanation of the drawings and the various embodiments of the invention will be offered. FIG. 1 shows a tone-control circuit of known design built up of an operational amplifier, controlled at its non-inverting input point (5). The operation of the circuit relies on the understanding that if potentiometer P is turned to its non-inverting input point 2, the oscillating circuit in series consisting of L elements will at the resonance frequency present a low impedance and consequently shunt the resistance R inserted between the non-inverting input point 2 and the earth. As a result of this shunting process the transmission characteristic will present a minimum at the resonance frequency. If potentiometer P is turned to mid-position, owing to the bridge type circuitry the oscillating circuit in series will be ineffective, i.e., the transmission characteristic will become a straight line. On the other hand if potentiometer P is turned towards inverting point 1, the minimum resistance of the oscillating circuit in series at the resonance frequency will shunt resistance R, connected between the inverting input point 1 and the earth and consequently owing to the known properties of the operation amplifier a maximum gain will be indicated. If e.g. R R R R l kohm, P l kOhm, L O,l H and C I00 F; the transmission characteristic of the low-tone control circuit has been plotted in the diagram in FIG. 2. Curve I will manifest itself with the slide of potentiometer P at the inverting input point 1, curve II has been plotted for the slide in mid-position, curve III is associated with the slide at the non-inverting point 2.

A deficiency of this known circuitry is the absence of band limitation below the lower limit of the useful audio frequency band, i.e., below about 40 Hz. Consequently signals of extremely low frequencies and even direct current will be transmitted with the same gain, i.e., a gain of unity. Thus the noise output below the lower limit of the band of the signal sources connected before the circuit will also be transmitted with a gain of unity, a circumstance which will interfere with the gain of the useful signals.

Another deficiency of the known method is the generally extreme tolerance of the capacity of the highcapacity condenser C and of its return loss at acceptable costs. Consequently the scattering of the values of both the resonance frequency and the unevenness producible at the resonance frequency will be very high.

All these deficiencies have been eliminated by the circuitry according to the invention and displayed in FIG. 3. Instead of condenser C in FIG. 1 now resistance R is connected in series with inductance L, while instead of resistance R, a differentiating circuit is connected between the controlling signal source and the non-inverting input point. The circuit is built up of condenser C and resistance R At a given frequency, with the slide of potentiometer P at the inverting point 1, the circuit will present a maximum emphasis. Below this frequency the characteristic will drop with a slope of 6 dB/octave, so that at d.c. attenuation will become infinite. A characteristic of this type has been plotted in the graph in FIG. 4 (curve I) for R R R R l kOhm; P =1 kOhm; L= 0.8 H; R 82 ohms; C =5 ,uF. With the slide of potentiometer P, in midposition (curve II) the characteristic of the gain will outside the useful band drop in like way monotonously, with a slope of -6 dB/octave. If the slide of potentiometer P is in the neighbourhood of the non-inverting point 2, the characteristic will in like way drop with a slope of -6 dB/octave (curve III). Since the highcapacity condenser C has been discarded (FIG. I) and the emphasis has been adjusted by the ratios of the impedance, a circuit of improved accuracy may be designed at lower costs.

As taught by experience in the majority of the field uses the ohmic resistance of the winding of an inductance L is sufficiently high to permit the elimination of resistance R Again experience shows that an operational amplifier of an open-loop gain of about A 5 dB will not be saturated in the d.c. sense even when the slide of potentiometer P is in the neighbourhood of the inverting input point 1, i.e., the ohmic resistance of inductance L in the d.c. sense shunts resistance R so defeating the statement made in the literature (5).

Hence with the method according to the invention presented in FIG. 3 the drawbacks and deficiencies of the known method as shown in FIG. 1 have been eliminated. v

If instead of condenser C an end of the secondary winding of an appropriately calculated transformer of primary inductance is connected to the free terminal of resistance R while the other end of the secondary winding is terminated on the earth, band limitation may be guaranteed with similar results.

In FIG. 5 a known design is shown of a high-tone control circuit built up with an operational amplifier and controlled at its non-inverting point (5). If potentiometer P is in mid-position, condenser C will be ineffective, so that the characteristic will be a straight line (curve II in FIG. 6). On the other hand if by turning the potentiometer condenser C will become connected between the inverting input point 1 and the earth and above a definite frequency a characteristic rising with a slopeof 6 dB/octave will be obtained (curve I, in FIG. 6); while if the slide of potentiometer P is turned to the non-inverting input point 2 a dropping characteristic of a slope of 6 dB/octave will present itself (curve III). The characteristics shown in FIG. 6 have been obtained for values ofC 0.2 uF, R R, R R =1 kOhm, P2 I I A drawback of the known design is that at the upper limit of the useful audio frequency band, i.e., for signals of a frequency of more than 16 kHz, the gain of the circuit rises with a slope of 6 dB/octave, and its loss with a slope of 6 dB/octave, whereas with the slide of potentiometer P in mid-position gain and loss will in the wide band become constant for frequencies higher than the upper cut-off frequency of the audio-frequency band. Consequently as a band limitation will not exist unless the slide of potentiometer P is in the neighbourhood of the non-inverting point 2, the known design will in other positions of the slide of potentiometer P amplify the disturbing noise voltages arising in the section of the electro-acoustic chain preceding the circuit quite appreciably. Moreover owing to the low, yet inevitable non-linear properties of the amplifier the components of the noise voltage of out-of-band frequencies will because of cross-modulation tend to increase the noise voltage within the useful band.

This deficiency of the known design may conveniently be abated by a method adopted by the present invention consisting in theconnection of an appropriately calculated resistance R between condenser C and the earth (FIG. 8). As is made clear by the graph in FIG. 7 the method falls short of offering a perfect solution, since if the slide of potentiometer P is close to the inverting input point 1 (curve I), for signals of a frequency outside the useful band the gain will no longer rise monotonously, but will remain constant on a given level. On the other hand if the slide of potentiometer P is at the non-inverting input point 2, outside the useful band the gain will become constant in a similar manner. Finally with the slide of potentiometer P in midposition there will be no change at all.

A design independent of the position of the slide of potentiometer P presenting band-limiting properties will be achieved when in addition to what has been proposed above a condenser C is connected parallel to resistance R i.e., an integrating circuit RC is connected between the controlling signal source and the noninverting point (FIG. 8). With the values properly chosen, i.e., R R, R R I kOhm, P I kOhm, R 0.15 kOhm, C 0.2 uF, C 0.016 uF, the characteristics displayed in FIG. 9 will be obtained. Here the curves are distinguished by the same symbols as before.

As may be inferred from the diagram, band limitation will in each case take place above the upper cut-off frequency of the audio frequency band irrespective of the position of the slide of potentiometer P The same result may also be guaranteed if instead of a condenser of capacity C another of a capacity C is connected parallel to resistance R (FIG. 10).

have been connected. The network formed of resis-' tance R and condenser C is terminated between the. common point of the resistances in series and the earth point. By the proper choice or adjustment of the resistances R and R at a given frequency emphasis or cutting may be brought about in the transmission characteristic.

The circuitry shown in FIG. 11 may be extended by connecting a chain formed of ohmic resistances between the inverting and non-inverting input points. The interconnected points of the resistances are terminated upon contact with a section switch and by connecting a network formed of a resistance R and condenser C to the contacts of the section switch a precision tone control circuit will be obtained.

A lowand high-tone control circuit may also be obtained by connecting the circuits described above, one after the other.

However, experience shows the lowand high-tone control circuits shown in FIGS. 3, 8 or 10, each including separate amplifiers and band limitation outside the audio frequency band, may be built up by using a single operational amplifier, without any undesirable result, by means of the circuitry in FIG. 12. In the latter circuit the degree of transmission of lowor high tones may be controlled independently of each other by means of potentiometers P and P respectively. The characteristics are shown in FIG. 13, it being understand that in the circuitry according to FIG. 12 R, R R R lkOhm, C 0.2 F, C 5 uF, C 0.016 uF, R 47 ohms, R 0.15 kOhm, L 0.8 H.

By carrying through other measures, with the properties of the circuitry in FIG. 12, the circuitry may be used for the compensation of the local unevennesses, peaks and depths, of the transmission characteristic. A circuitry suited for this purpose is shown in FIG. 14, where by means of the loss-type oscillating circuits in series, L C R further L C R and the potentiometers, respectively P and P attached to them at two optionally selected frequencies in a manner controlled independently of each other peak and/or hole type unevennesses may be brought about. The bandwidths of the unevennesses may be controlled one by one by the choice of the resistances R and R As taught by experience with the use of a single operational amplifier five local unevennesses (peak and/or hole) controlled separately from one another may be brought about on the transmission characteristic. The characteristics are shown with the potentiometers in extreme and midposition, with the notations used earlier, in FIG. 15, it being understood that L L L L 0.1 H, C 16.2 nF, C 12.5 nF, C 42.2 nF, C 0.309 ,uF, R 0.1 kOhm, R 0.22 kOhm, R 0.22 kOhm, R 0.22 kOhm, while all potentiometers have a resistance of one kOhm.

In oscillating circuits in series, when the bandwidth has been specified, the lateral slope cannot be increased beyond a given limit. Therefore if for a given,

relatively large bandwidth, eg one of one third of an octave a larger lateral slope is essential, the method according to FIG. 16 will produce the desired result. As a matter of fact if the oscillating circuit in series built up of elements L C R and the other oscillating circuit in series, built up of elements L C R have been designed for a sufficiently low loss, with the resonance frequencies of the oscillating circuits selected in a manner slightly differing from one another and connected in parallel to the slide of potentiometer P by applying the earlier notations, the characteristics shown in FIG. 17 will be obtained for the different positions of the slide of potentiometer P The characteristics presented by way of example have been obtained for elements of the following values: R R, R R l kOhm, R l kOhm, R R 120 ohms, C 0.032] uF, C 0.0265 ILF, L L 0.7 H.

Earlier it has already been made clear that as taught by experience with the use of a single operational amplifier as many as five local unevennesses controllable in a manner independent of one another, i.e., peaks and/or holes, high and low-tone emphases and/or cuts may be brought about in the transmission characterisno.

If a larger number of unevennesses have to be provided in the transmission characteristic, the inputs of the tone control circuit designed as detailed above have to be connected in parallel, whereas the outputs of the circuits should be interconnected by resistances in series and then terminated on one end of a common resistance. The other end of the common resistance is in this case terminated on the earth point. An embodiment of this method is shown in FIG. 18, which is selfexplanatory.

An improved variant of the method of FIG. 18 is shown in FIG. 19. Here the inputs of the tone control circuits are again connected in parallel, whereas their outputs are through the intermediary of resistances in series terminated on the inverting point of another operational amplifier of differential input. There is an ohmic resistance connected between the non-inverting and the earth points, and also between the inverting point and the output of the operational amplifier. The output of this newly created operational amplifier is at the same time the output of the whole circuit.

What we claim is:

1. In a tone control circuit for high and/or low tone control, containing at least one operational amplifier of differential input controlled at its non-inverting point, with an ohmic resistance connected between the inverting point and earth point and also between the noninverting point and earth point of the amplifier, further with an ohmic resistance connected between the inverting point and the output point of the operational amplifier, a potentiometer connected between the inverting and non-inverting points, the slide of said potentiometer being connected through a reactance to the earth point, and an ohmic resistance connected between the controlling signal source of the operational amplifier and the non-inverting point the improvement comprising an ohmic resistance connected in series with the reactance. inductance and condenser connected between the slide of the potentiometer and the earth point, and an RC circuit connected between the controlling signal source and the non-inverting point.

2. A tone control circuit according to claim 1, characterized in that the resistance in series with the inductance is formed by the ohmic resistance of its own of the winding constituting the inductance.

3. A tone control circuit according to claim 1, characterized in that between the inverting and noninverting points of the operational amplifier at least two potentiometers are connected in parallel, between the slide of the one and the earth point the connection in series of an inductance and an ohmic resistance, between the slide of the other and the earth point the connection in series of a condenser and an ohmic resistance being connected, and a differentiating and integrating RC circuit connected between the controlling signal source and the non-inverting point.

4. A tone control circuit according to claim 1, and a further potentiometer connected in parallel between the inverting and non-inverting points, between the slide of which potentiometer and the earth point at least one network comprised of a resistance, condenser and inductance in series is connected.

5. A tone control circuit according to claim 1, which includes at least two operational amplifiers, together with the associated circuit, which are connected in parallel between the controlling signal source and the output of the circuit in a manner such that the outputs of the operational amplifiers are terminated on the common output by inserting an ohmic resistance, while an ohmic resistance is connected between the common output point and the earth point.

6. A tone control circuit according to claim 5, in which instead of the ohmic resistance connected between the commoned output point of the operational amplifiers and the earth point the inverting input of a further operational amplifier with a differential input is terminated on the common output point, an ohmic resistance connected between the non-inverting input and the earth point, and an ohmic resistance connected between the inverting input and the output of the operational amplifier, at the same time constituting the output of the circuit as a whole.

7. A tone control circuit according to claim 1 in which at least one potentiometer connected between the inverting and non-inverting points of the operational amplifier is replaced by two resistances of different values in series and on the common point of these resistances a circuit consisting of a reactance and ohmic resistance, with its other end connected to the earth.

8. In a tone control circuit for high and/or low tonecontrol, containing at least one operational amplifier of differential input controlled at its non-inverting point, with an ohmic resistance connected between the inverting point and earth point and also between the noninverting point and earth point of the amplifier, further with an ohmic resistance connected between the inverting point and the output point of the operational amplifier, a potentiometer connected between the inverting and non-inverting points, the slide of said potentiometer being connected through a reactance to the earth point, and an ohmic resistance connected between the controlling signal source of the operational amplifier and the non-inverting point the improvement comprising an ohmic resistance connected in series with the reactance, inductance and condenser connected between the slide of the potentiometer and the earth point, an ohmic resistance connected between the signal source and the non-inverting point, and a condenser connected in parallel to the last-named ohmic resistance and connected between the inverting point and the output of the operational amplifier.

9. In a tone control circuit for high and/or low tone control, containing at least one operational amplifier of differential input controlled at its non-inverting point, with an ohmic resistance connected between the inverting point and earth point and also between the noninverting point and earth point of the amplifier, further with an ohmic resistance connected between the inverting point and the output point of the operational amplifier, a potentiometer connected between the inverting and non-inverting points, the slide of said potentiometer being connected through a reactance to being connected to the controlling signal source. 

1. In a tone control circuit for high and/or low tone control, containing at least one operational amplifier of differential input controlled at its non-inverting point, with an ohmic resistance connected between the inverting point and earth point and also between the non-inverting point and earth point of the amplifier, further with an ohmic resistance connected between the inverting point and the output point of the operational amplifier, a potentiometer connected between the inverting and non-inverting points, the slide of said potentiometer being connected through a reactance to the earth point, and an ohmic resistance connected between the controlling signal source of the operational amplifier and the non-inverting point the improvement comprising an ohmic resistance connected in series with the reactance, inductance and condenser connected between the slide of the potentiometer and the earth point, and an RC circuit connected between the controlling signal source and the noninverting point.
 2. A tone control circuit according to claim 1, characterized in that the resistance in series with the inductance is formed by the ohmic resistance of its own of the winding constituting the inductance.
 3. A tone control circuit according to claim 1, characterized in that between the inverting and non-inverting points of the operational amplifier at least two potentiometers are connected in parallel, between the slide of the one and the earth point the connection in series of an inductance and an ohmic resistance, between the slide of the other and the earth point the connection in series of a condenser and an ohmic resistance being connected, and a differEntiating and integrating RC circuit connected between the controlling signal source and the non-inverting point.
 4. A tone control circuit according to claim 1, and a further potentiometer connected in parallel between the inverting and non-inverting points, between the slide of which potentiometer and the earth point at least one network comprised of a resistance, condenser and inductance in series is connected.
 5. A tone control circuit according to claim 1, which includes at least two operational amplifiers, together with the associated circuit, which are connected in parallel between the controlling signal source and the output of the circuit in a manner such that the outputs of the operational amplifiers are terminated on the common output by inserting an ohmic resistance, while an ohmic resistance is connected between the common output point and the earth point.
 6. A tone control circuit according to claim 5, in which instead of the ohmic resistance connected between the commoned output point of the operational amplifiers and the earth point the inverting input of a further operational amplifier with a differential input is terminated on the common output point, an ohmic resistance connected between the non-inverting input and the earth point, and an ohmic resistance connected between the inverting input and the output of the operational amplifier, at the same time constituting the output of the circuit as a whole.
 7. A tone control circuit according to claim 1 in which at least one potentiometer connected between the inverting and non-inverting points of the operational amplifier is replaced by two resistances of different values in series and on the common point of these resistances a circuit consisting of a reactance and ohmic resistance, with its other end connected to the earth.
 8. In a tone control circuit for high and/or low tone control, containing at least one operational amplifier of differential input controlled at its non-inverting point, with an ohmic resistance connected between the inverting point and earth point and also between the non-inverting point and earth point of the amplifier, further with an ohmic resistance connected between the inverting point and the output point of the operational amplifier, a potentiometer connected between the inverting and non-inverting points, the slide of said potentiometer being connected through a reactance to the earth point, and an ohmic resistance connected between the controlling signal source of the operational amplifier and the non-inverting point the improvement comprising an ohmic resistance connected in series with the reactance, inductance and condenser connected between the slide of the potentiometer and the earth point, an ohmic resistance connected between the signal source and the non-inverting point, and a condenser connected in parallel to the last-named ohmic resistance and connected between the inverting point and the output of the operational amplifier.
 9. In a tone control circuit for high and/or low tone control, containing at least one operational amplifier of differential input controlled at its non-inverting point, with an ohmic resistance connected between the inverting point and earth point and also between the non-inverting point and earth point of the amplifier, further with an ohmic resistance connected between the inverting point and the output point of the operational amplifier, a potentiometer connected between the inverting and non-inverting points, the slide of said potentiometer being connected through a reactance to the earth point, and an ohmic resistance connected between the controlling signal source of the operational amplifier and the non-inverting point the improvement comprising an ohmic resistance connected in series with the reactance, inductance and condenser connected between the slide of the potentiometer and the earth point, one end of the secondary winding of a transformer being terminated on the free end of the ohmic resistance towards the contrOlling signal source, the other end of said winding being terminated on the earth point, the primary winding of the transformer being connected to the controlling signal source. 